It is a frequent practice in the semiconductor industry to apply a protective polymer coating over a finished or semi-finished product, e.g., completely configured printed circuit (PC) boards with integrated circuits (ICs) mounted thereon. A frequently used protective polymer is parylene, which provides a conformal coating that is easily applied. In many cases it is necessary to remove the coating at some later time, e.g., to make changes or repairs on the circuit board. Though its ease of application and its coverage capabilities make parylene a desirable coating material, it is extremely difficult to remove.
Parylene is a generic name for members of a series of polymerized paraxylylenes whose basic member is poly-para-xylylene, commonly known as parylene-N. The monomer of poly-para-xylylene consists of a benzene ring bonded to two methyl groups to create a linear molecule. A second member of the series is poly-monochloro-para-xylylene, known as parylene-C. Poly-monochloro-para-xylylene is a variation of poly-para-xylylene wherein the variation consists of a single chlorine atom substituting for one of the aromatic hydrogens in the benzene ring of the monomer molecule. A third member of the series is poly-dichloro-para-xylylene, commonly known as parylene-D. The member of the series known as parylene-E (poly-ethyl-para-xylylene) has an ethyl group in place of the chlorine in poly-monochloro-para-xylylene. More generally, parylene-E contains an alkyl group substituted for the ethyl group. Additionally, fluorinated parylenes are commonly referred to as parylene-F. Throughout this text, the term parylene will be used in a generic sense and will refer to poly-para-xylylene, its derivatives, and co-polymers.
Poly-para-xylylene and poly-monochloro-para-xylylene are generally deposited with substantially the same process. Exemplarily, either dimer is vaporized at approximately 250.degree. C. The dimer is then pyrolized at about 680.degree. C. into a monomer, which is allowed to diffuse at room temperature into a deposition chamber where it condenses and polymerizes on the surface of everything in the chamber in a conformal manner. The low temperature deposition and complete coverage properties of parylene make it very useful as a protective coating. Parylene as a protective coating for electronic printed circuit boards is especially advantageous. See, for instance, R. Olson, Proceedings of the 17th Electrical/Electronics Insulation Conference, pp. 288-290, 1985, and U.S. Pat. No. 4,123,308, both incorporated herein by reference.
Poly-monochloro-para-xylylene has a lower water absorption rate, lower coefficient of thermal expansion, and generally forms a more pinhole free film than poly-para-xylylene. In addition, the inclusion of one chlorine atom on each benzene ring of the polymer chain makes poly-monochloro-para-xylylene extremely resistant to solvents. Though this makes parylene-C a very good protective coating, it also makes the repair of assemblies and/or subassemblies difficult if the coating must be removed.
Parylene-E, on the other hand, can be dissolved, though with some difficulty, in some solvents such as xylene, toluene, hexane, methylene chloride, and chloroform. The mixing of parylene-E and parylene-C allows for the engineering of the solvent resistance of a coating while retaining some of the protective benefits of the moisture and insulation properties of parylene-C. Methods of coating removal, other than mixing with sufficient parylene-E to permit solvent removal, are available. These include abrasion, chemical-aided removal, and plasma etching in an oxygen barrel reactor; however, these methods have also not proved completely satisfactory.
Use of abrasion techniques runs the risk of damaging coated electronic and mechanical parts adjacent to the coating being removed, and of generating dirt and dust that may be difficult to remove. Chemical-aided parylene removal methods, such as those used in U.S. Pat. No. 4,734,300, still require physical means to remove a coating from an article, subjecting the article to possible damage from the physical means applied. The use of a chemical, for instance, tetrahydrofuran in the aforementioned patent, can result in the chemical attack of coatings and components adjacent to and under the coating being removed. Plasma etching in an oxygen barrel reactor typically is slow requiring long processing times. For instance, U.S. Pat. No. 4,123,308 discloses that parylene exposed to an oxygen plasma is typically etched at the rate of 1000 .ANG. per minute. This rate, in many cases, is too low for removing parylene from PC boards in a manufacturing environment. In addition, ions in the plasma can cause damage to electronic components, exemplarily due to electrostatic discharge (ESD) that can result from bombardment by the energetic ions.
Plasma etchers have been developed that separate the plasma generating section from the reaction chamber in which the etching takes place. This allows for the generation of plasma discharge products, a gas of reactive atoms and molecules, without electrons and ions bombarding the body being etched. In addition, the reaction from the contact of the plasma discharge products with the body is downstream from the plasma source. Some configurations of this type of "downstream" plasma etcher have used microwave generators as the plasma source to more efficiently couple energy into the plasma. Such microwave plasma etchers are described, for example, in U.S. Pat. No. 4,673,456, U.S. Pat. No. 4,138,306, U.S. Pat. No. 4,175,235, and U.S. Pat. No. 4,776,923. U.S. Pat. No. 4,776,923 also describes a method in which ultraviolet radiation generated in the plasma generating section is prevented from impinging on the body by the use of a bent path connecting the plasma generating section with the reaction chamber. Plasma etchers, including the types described in the aforementioned patents, have been used for the removal of SiO.sub.2, Si.sub.3 N.sub.4, photoresists, and polyimide from silicon wafers using a variety of gases such as O.sub.2, H.sub.2, N.sub.2 O, CF.sub.4, NF.sub.3, and SF.sub.6 and mixtures thereof.
Studies have shown that the etch rate for polyimide and photoresists from silicon wafers is increased by addition of N.sub.2 O, CF.sub.4, or SF.sub.6 to an oxygen gas flow in a plasma etcher. As the percentage of oxygen in the flow is decreased (percentage of the additive is increased), the etch rate typically increases to a maximum. Beyond the maximum etch rate, further decreasing the percentage of oxygen in the gas flow typically results in a rapidly decreasing etch rate. As pointed out by M. A. Hartney et al., Journal of Vacuum Science and Technology, B, Vol. 7, No. 1, pp. 1-13, 1989, in a CF.sub.4 /O.sub.2 gas flow the maximum etch rate for photoresists and polyimides usually occurs in the range of 20% to 30% CF.sub.4, with a sharp peak about this maximum. Similarly, etching polyimides in a SF.sub.6 /O.sub.2 gas flow exhibits a sharp maximum etch rate at about 5% SF.sub.6 (see, for instance, Emmi, F. et al., Proceedings of the Fifth Symposium on Plasma Processing , Vol. 85-1 of the Electrochemical Society, pp. 193-205, 1985). The percentage of the CF.sub.4 or the SF.sub.6 associated with the maximum in the polyimide etch rates has been determined to be, among other factors, a function of substrate temperature, gas flow rate, and generator power (applicable to either RF or microwave generators). Studies of the etching of polyimides and photoresists can be found in a series of articles in the Proceedings of the Fifth Symposium on Plasma Processing Vol. 85-1 of the Electrochemical Society, 1985 (for instance, Emmi, F. et al., pp. 193-205, Robinson, B. et al., pp. 206-215, Yogi, T. et al., pp. 216-226, and Charlet, B. et al., pp. 227-234).
Parylene is a unique material, being one of the few polymers capable of forming a conformal coating that is truly solvent resistant. In addition, unlike polyimide, parylene is a semi-crystalline material (i.e. it has a well defined melting temperature). As previously mentioned, several techniques have been applied to remove parylene coatings, but none has proven totally satisfactory.
In view of the desirability of parylene as a coating material, a method for quickly removing parylene from a body or selected areas of a body, while causing substantially no damage to the body or subassemblies of the body, would be of great significance. This application discloses such a method.